1. Field of the Invention
The present invention relates to a semiconductor device having improved withstanding voltage characteristics.
2. Description of the Related Art
In a conventional semiconductor device such as a semiconductor integrated circuit (IC), a conductive layer called a field plate is frequently formed so as to improve the withstanding voltage characteristics between a substrate and a diffusion region formed therein.
FIG. 4 is a sectional view of a semiconductor device having the field plate described above. N-type diffusion region 31 is formed in p-type silicon semiconductor substrate 30. Field plate 33 consisting of polysilicon or the like is formed through insulating layer 32 on a substrate surface near the end of diffusion region 31. The potential of field plate 33 is set at the same value as that of diffusion region 31. When field plate 33 is formed, depletion layer 34 extending from diffusion region 31 in the substrate is spread near the end of field plate 33, thereby improving the withstanding voltage characteristics between substrate 30 and diffusion region 31.
In a conventional IC, however, another wiring layer 36 is often formed through insulating layer 35 on field plate 33, as shown in FIG. 4. In this case, an electric field is concentrated near the end of field plate 33 by the influence of a difference between the potentials of wiring layer 36 and field plate 33. Therefore, the withstanding voltage between substrate 30 and diffusion region 31 is considerably decreased as compared with a case wherein wiring layer 36 is not arranged.
The thicknesses of insulating layers 32 and 35 may be increased as a countermeasure against the above problem. However, the process and pattern are limited in this method. For example, a wiring step coverage tends to be poor at corners having a thin insulating film. In addition, variations in thickness undesirably cause large variations in withstanding voltage characteristics.
When the thickness of the insulating layer is increased, cracks occur in the insulating layer and hence wiring layer 36 formed thereon. Therefore, this method has a disadvantage in reliability.
As has been described above, in a semiconductor device having a field plate, variations in withstanding voltage characteristics occur, thereby degrading reliability of the device.